If it's just regular bumps and you're not gaming the system (other than picking flights that are likely to ask for VDB, which is starting to be a lot of flights...), I agree that it's unlikely.

I flew at the holidays for 3 years in a row on a single ticket purchase from America West without any special attempt to get VDB. I bought an RT ticket one year, they were overbooked on the return flight so I took VDB. I used the voucher to buy my holiday ticket the next year, and again got VDB on the return. Lather, rinse, repeat. Nobody complained.